In general, any drive connection in a mechanical system exhibits some degree of compliance, i.e. a tendency to yield or bend under load, within the elastic limit of the material, or materials, of the components making up the connection. As a result of this compliance, a driving force exerted at one end of the connection causes the connection to stretch, bend, and/or twist, depending upon the nature of the connection, in such a manner that the driving force will be slightly out of phase with a corresponding reaction of a driven element at the opposite end of the connection, due to inertia of the driven component which must be overcome in order for the driving force to cause a motion of the driven element consistent with the motion of a driving element applying the driving force.
Under certain circumstances, depending upon construction of the system, compliance in the connection will cause an undesirable oscillating or resonant motion to be set up between the driving and driven elements.
Such oscillating behavior is sometimes observed in a system having an engine connected to an engine testing dynamo through a connection including an in-line torque sensor. Such torque sensors typically include a resilient element operatively joining an input element and an output element of the torque sensor. The resilient element allows the input and output elements to twist slightly, with respect to one another, in response to torque being transmitted through the torque sensor. This twisting can be measured and used to determine the torque being transmitted by the coupling.
During an increase and/or decrease in torque, however, the resilient element may cause the system to oscillate as energy is alternately stored and released in the resilient element, until equilibrium is achieved. Such oscillation can be damaging or otherwise detrimental to operation of the system and its components. It is desirable, therefore, to provide an apparatus and method for estimating such behavior, and for controlling the system in such a manner that the undesirable oscillatory or resonant behavior is precluded and/or held within acceptable bounds. It is also highly desirable, in some circumstances, to provide for such control without having sensors located at the driven element, i.e. at the dynamo in the example given above, in order to remove complexity and cost and to improve reliability of the system.
In some systems, oscillating or resonant behavior takes a form known as stick-slip behavior. Stick-slip behavior refers to an undesired intermittent form of motion that sometimes occurs between relatively moving parts where the coefficient of kinetic friction between the parts is less than the coefficient of static friction between the parts. Contacting surfaces of the parts will stick to one another until a driving force, being exerted on one of the parts by a drive element to cause relative movement between the parts, reaches a value high enough to overcome the static frictional force between the contact surfaces.
Due to the fact that the static coefficient of friction is higher than the kinetic coefficient of friction, once the static friction force is overcome by the driving force, the contact surfaces of the parts will tend to move freely and rapidly with respect to one another.
Because there is an inherent springiness (compliance) in the drive element applying force between the parts, the drive element will tend to stretch or compress, or wind up, as force is applied to the movable part while the contact surfaces are being held in contact by the static friction force. Once relative motion occurs, this compression, tension, or winding-up of the drive element will cause rapid movement between the parts, to release the energy stored in compression, tension or wind-up of the drive element. Once the stored energy is released, however, through rapid relative movement between the parts, the relative velocity between the contact surfaces will drop to the point that the static friction force will once again cause the parts to stick to one another, and thereby preclude further relative motion, until sufficient compression, tension, or wind-up of the drive element once again occurs, to overcome the static frictional force and cause slipping of the contact surfaces relative to one another.
Such stick-slip behavior is known to sometimes occur in metal working equipment, for example, where a drill bit or milling cutter must be driven by a power source located some distance from the point at which material removal is occurring, such that the drill bit or cutter must have a long shank, and/or be connected to a long drive shaft.
Stick-slip behavior is also sometimes encountered in machinery used in drilling for, or pumping fluids, such as gas, water, or oil, out of the ground. In such applications, long shafts, having lengths of hundreds or thousands of feet, may connect a drilling or pumping apparatus located far below ground level to a shaft drive mechanism located above ground level. Such long shafts have considerable inherent springiness, both axially and radially. This considerable springiness allows a significant amount of energy to be stored in the shaft, if the underground components stick to one another, such that when the torsional force due to wind-up of the shaft becomes high enough to cause the underground parts to break free from one another, they will slip relative to one another at a very high rotational speed, until the energy stored in the shaft is dissipated.
In addition to placing significant undesirable strain on the working components of the system, stick-slip operation of a pump also will substantially reduce the pumping capacity of the pump. While the parts are stuck to one another no relative motion or pumping is occurring, and during a portion of the stick-slip cycle in which the parts are moving very rapidly with respect to one another, pumping may also not be occurring.
Stick-slip operation, and its detrimental effects, is further discussed in a United States Patent Application No. US 2004/0062658 A1, published Apr. 1, 2004, to Beck, et al., assigned to the assignee of the present invention, the disclosure and teachings of which are incorporated herein in their entirety.
Prior approaches to dealing with a system exhibiting stick-slip behavior, have sometimes utilized sensors located adjacent to the contacting surfaces subject to stick-slip behavior. In oil pumping operations, for example, this has sometimes required placement of sensing equipment a mile or more below the earth's surface and making connections to a controller located above ground. Such sensors tend to be quite expensive to produce and maintain, and are prone to failure due to the hostile environment in which they are located. Should repair of the sensing elements be required, significant interruption to the pumping process is incurred, in pulling the sensing unit back up to the surface of the ground where it can be repaired and/or replaced.
It is highly desirable, therefore, to provide an improved method and/or apparatus for estimating and controlling undesirable oscillatory or resonant behavior in a system prone to such behavior, and particularly in systems which may be prone to stick-slip behavior. It is also desirable to provide an apparatus and/or method for controlling such systems with a minimal number of transducers, and preferably without the necessity for having such transducers located near a driven element of the system.